A large number of processes have been proposed for removing sulfur dioxide from flue gases. The stage of development of these processes ranges from preliminary tests of conceptual designs to full scale tests in commercial plants. Among the latter, the removal of sulfur dioxide using aqueous slurries of calcium carbonate or calcium oxide has been a preferred process as a result of the many years of commercial testing as well as an abundant supply of relatively cheap reactive agent.
The process is severely handicapped, however, because it has not been possible to recover spent chemicals and is thereby confronted with a difficult disposal problem. One aspect of this problem is the need to haul spent chemicals to a suitable dumping area where the chemicals will not result in water and air pollution. Thus the process does not necessarily abate pollution, it only transfers the problem from one location to another.
Another negative factor is plugging and corrosion encountered in various areas of the gas scrubber elements during operation. The net result is that the operating time efficiency for such scrubbers has been extremely low in almost all cases where such scrubbing processes have been used and no process has given continuous runs exceeding a few weeks' duration. Most runs have been short and have interfered with proper operation of electric power generators. Such difficulties, added to the sludge disposal problem, have given the scrubbing process a poor reputation with a resulting negative attitude by the public utilities towards all sulfur dioxide removal processes.
It must be noted, however, that intensive effort is being applied to overcome technological problems associated with sulfur dioxide removal. A promising variation, in the pilot plant stage, is the double alkali process which uses a solution of sodium hydroxide in the scrubber to derive a solution of sodium sulfite or bisulfite and to react the latter with an alkaline earth oxide, such a calcium oxide, to recover sodium hydroxide for re-use in the scrubber with the concomitant conversion of calcium oxide to calcium sulfite. This process solves many of the problems concerning scrubber operation, but does nothing for the calcium sulfite disposal problem.